Packaging semiconductor integrated microcircuits generally entails enclosing the microcircuit chip in one of two package types; ceramic or plastic. High-reliability or high power applications of microcircuits generally use ceramic packages. Ceramic packages can provide a hermetic seal preventing exposure of the microcircuit chip within the package to moisture or other contaminants. Ceramic packaging, however, has high material costs, high assembly costs, and high labor costs. High volume, low cost, commercial applications of microcircuits, therefore, generally do not use ceramic packages.
In contrast, plastic packaging provides the brunt of the low cost, high volume packages used in commercial electronics today. Unfortunately, the inability of plastic to effectively dissipate high levels of power from a microcircuit chip makes plastic packages unsuitable for some applications. Additionally, forming a plastic package requires sealing the microcircuit chip in a molding compound at an elevated temperature. Either the molding temperature or molding compounds may not be compatible with the microcircuit chip.
Some classes of semiconductor microcircuits require a window or lens above the microcircuit chip for proper operation. For example, in erasable programmable read-only memories (EPROMS) and programmable array logic (PAL) devices, ultra-violet (UV) light erases or clears the device.
A new type of microcircuit that transmits or reflects light from the surface of the chip through a window or lens in the package has been developed. A Digital Micromirror Device (DMD) reflects light transmitted through the package window. DMDs have been developed for use in, among other applications, high definition television (HDTV). Details on DMDs may be found in U.S. Pat. No. 5,061,049, issued to Hornbeck from patent application Ser. No. 07/582,804, and assigned to Texas Instruments Incorporated. U.S. Pat. No. 5,061,049 is expressly incorporated by reference for all purposes.
Proper operation of devices that transmit or reflect light through a package window require nearly perfect parallelism between the lens or window and the surface of the microcircuit chip. Failing to achieve parallelism between the lens and chip surface may result in unacceptable reflection or refraction of light from the chip surface. Additionally, because the chip may reflect substantial amounts of light energy, the chip and package must dissipate power efficiently. The inability to dissipate power efficiently may cause the temperature of the chip to exceed a level where it may be damaged.
To date, light reflecting and transmitting microcircuits have been primarily packaged in ceramic cavity packages. Because of the rigid structure of a ceramic package, achieving parallelism between the chip surface and the lens or window is possible. As previously noted, ceramic packages dissipate power effectively and, therefore, meet this additional requirement for light transmitting or reflecting microcircuits. Unfortunately, the expense of ceramic cavity packages makes them unsuitable for low cost systems using light transmitting devices like DMDs.
Low cost, high volume plastic packaging has thus far proven unacceptable for DMDs. Most previously developed plastic packages lack sufficient power dissipating capability for use with DMDs. Additionally, the molding temperature for most plastic compounds (approximately 180.degree. C.) may damage some microcircuits, including DMD devices. Therefore, using plastic packages for these devices requires pre-forming a plastic cavity package in which the chip is later placed. Previously developed techniques for pre-forming plastic cavity packages do not provide or ensure parallelism between the lens or window covering the cavity and microcircuit chip surface.